1. Field of the Invention
The present invention relates to proppant testing methods and apparatuses.
2. Background of the Related Art
Oil and natural gas are produced from wells having porous and permeable subterranean formations. The porosity of the formation permits the formation to store oil and gas, and the permeability of the formation permits the oil or gas fluid to move through the formation. Sometimes the permeability of the formation is insufficient for economic recovery of oil and gas. In other cases, during operation of the well, the permeability of the formation drops to such an extent that further recovery becomes uneconomical. In such circumstances, it is common to fracture the formation and prop the fracture in an open condition using a particulate material known as a proppant.
Fracturing is usually accomplished by hydraulic pressure using fluids such as water, water viscosified with polysaccharides, and other polymers, crosslinked polysaccharides, or hydrocarbon-based, gel-like fluid. The hydraulic pressure is increased until cracks or fissures form in the underground rock. The proppant particles, which are suspended in this pressurized fluid, are forced into the cracks. The proppant particles prop open the formed cracks so the cracks remain open when the hydraulic pressure is reduced. The oil or gas is then able to flow through the interstices between the particles to increase economic recovery. The oil or gas flows to collection regions and is pumped to the surface.
A wide variety of materials are used as proppants. Typical proppant materials include sand, glass beads, and ceramic pellets. The proppant used during a fracturing operation may be selected according to the geological conditions of the formation to be fractured. The physical properties of a particular proppant material determine its effectiveness. The particle size of a proppant, for example, affects the permeability of the fractured formation, and the corresponding ability for hydrocarbons to flow through the proppant pack. The crush strength of the proppant provides an indication of how well the proppant resist the crushing subterranean forces while propping open a fracture. Over time, the pressure of the surrounding rock tends to crush the proppants, resulting in reduced particle size, which reduces permeability. Very fine proppant particles referred to as “fines” may result from crushing. Fines tend to migrate and plug the interstitial flow passages in the propped structure. These migratory fines drastically reduce the permeability, lowering the conductivity of the oil or gas.
The importance of the physical properties of proppants is well recognized in the industry. The American Petroleum Institute (API) has issued Recommended Practices for proppant testing. For example, API Recommended Practices RP-19C covers testing procedures for sand used in hydraulic fracturing operations. These Recommended Practices include testing procedures for determination of properties that include particle size and crush resistance. International Standard ISO 13503-2 “Petroleum and natural gas industries-Completions fluids and materials-Part 2: Measurement of properties of proppants used in hydraulic fracturing and gravel-packing operations” also governs this testing.
Proppants are commonly tested above ground, such as in a laboratory, under conditions of pressurized fluid flow in combination with a crushing level of force, to simulate subterranean formation conditions. To test a proppant under such conditions, a proppant sample is typically placed in the cavity of a pressure vessel and trapped within the cavity between a cavity wall and a piston. In this context, the proppant sample may be referred to as a “proppant pack.” The piston is driven against the proppant pack with a crushing level of force, often in combination with flowing pressurized fluid through the cavity and the proppant pack. The forces and pressures involved in proppant testing can result in the piston becoming stuck within the vessel. The piston must be removed from the vessel before the proppant can be removed for analysis. However, there is conventionally no easy way to free a stuck piston from a vessel.